1. Field of the Invention
The present invention relates to a method of manufacturing a toner.
2. Discussion of the Background
Toner images are formed by attaching toner to a latent electrostatic image formed on an image bearing member, transferring the latent electrostatic image to a recording medium and fixing the image thereon upon application of heat in an electrophotographic apparatus or an electrostatic recording apparatus. Full color images are generally formed by color reproduction using toners of four colors of yellow, magenta, cyan and black, i.e., each color image is developed with respective color toners and overlapped a top on a recording medium to form a color image followed by fixing upon application of heat.
Generally, these full color images do not reach the quality with which a user familiar with printing is satisfied and therefore are required to have quality of high fineness and high definition close to that of photographs or printing industry. Under this circumstance, it is known that a toner having a small particle diameter and a sharp particle size distribution is effective to improve the quality.
Electrostatic or magnetic latent images are typically developed with toner.
The toner for use in latent electrostatic images is generally colored particles which include a coloring agent, a charge control agent, and other additives in a binder resin. The methods of manufacturing this toner are typified into pulverization methods and chemical methods (for example, a spray drying method of adjusting spherical mother toner particle material by spraying and drying an organic solvent in droplet state in which binder resin components are dissolved, an o/w type emulsion method of adjusting spherical mother toner particle material by dispersing an organic solvent in which a binder resin component is dissolved in an aqueous medium in droplet form followed by removal of the solvent, a polymerization method, partial polymerization method, etc.). In the pulverization method, a coloring agent, a charge control agent, an offset prevention agent, etc., are melted, mixed and uniformly dispersed in a thermoplastic resin and the obtained toner composition is pulverized and classified to manufacture toner.
According to the pulverization method, toner having excellent characteristics in some degree can be manufactured but selection of toner material is limited. For example, the toner component obtained by melting and mixing is pulverized and classified by a machine affordable in terms of economy. Therefore, the melted and mixed toner composition is manufactured to be inevitably brittle. Thus, when the toner composition is pulverized for making particles, the obtained particles tend to have a wide particle size distribution so that fine particles having a particle diameter of 3 μm or smaller and coarse particles having a particle diameter of 10 μm are classified and removed. This results in an extremely low yield. Classification is conducted by using a typical classifier using air force in most cases. Such a classifier cannot avoid jump-in of particles having a relatively large particle diameter in comparison with the target particle diameter due to disturbance in air wind and thus coarse particles are unavoidably produced.
On the other hand, there are chemical methods such as a polymerization method or an emulsification dispersion method in which mother toner particles or particles containing binder resin are produced in an aqueous medium. As the polymerization method, a suspension polymerization method is known which conducts polymerization after adding a monomer, a polymerization initiator, a coloring agent, a charge control agent, etc., to an aqueous medium containing a dispersion agent to form oil droplets while stirring. In addition, an association method is also known in which particles obtained by using an emulsification polymerization or a suspension polymerization are agglomerated and adhered.
Toner having a small particle diameter with a sharp particle size distribution can be obtained without classification by such methods. However, while toner having a sharp particle size distribution is obtained without classification in some cases, coarse particles are occasionally produced by condensation, agglomeration or peeling of particles attached to the wall of pipe.
Known classifier is used to remove these coarse particles as in the case of the pulverization method. For example, a strainer is used to remove the coarse particles in the granulation process. Unexamined published Japanese patent application No. (hereinafter referred to as JOP) 2002-162772 describes an air wind classification technology by which the remaining amount of the coarse particles is reduced within 10 mg when 100 g of pulverized toner is screened with a screen having a 500 mesh. JOP H04-121112 describes a technology of screening powder in which balls are used to give vibration to a screen. As to the strainer, due to the limitation in terms of the productivity, the mesh of the strainer is inevitably coarse, i.e., 100 μm at minimum, which is more than ten times as large as the toner particle diameter. Thus, the coarse particles are not sufficiently removed.
Furthermore, removing coarse particles by classification using a decanter and the same classification process as in the pulverization method subsequent to drying are thinkable to remove coarse and fine powder. However, adding such a classification process is not preferable because the addition of such an unnecessary process will result in reduction in productivity. Also, that process is just a pneumatic classification simply using the weight difference or the weight difference enlarged by acceleration (typically angular acceleration by rotation power). Therefore, coarse particles having a particle diameter several times as large as the target particle diameter may jump in as described above and a small amount of the coarse particles is mingled in the manufacture product, which causes clogging of the gap in a development device, resulting in creation of non-development portion.
To solve these problems, for example, JOP H06-19201 describes a technology which regulates the form and the opening of a screen to prevent production of coarse particles in the screening process, adhesion of particles due to mechanical heating, and re-agglomeration of particles by van der Waals' forces. A multiple storied gyroshifter is used as a device having screens and performs screening by utilizing mechanical vibration or ultrasonic vibration. However, the opening is significantly larger than the target particle diameter so that the coarse particles are not sufficiently removed. Therefore, the coarse particles that have passed through the development process are not transferred because the coarse particles do not have a sufficient amount of charge and therefore contact with and fracture a cleaning blade when remaining toner is removed thereby. Furthermore, even when the coarse toner particles are transferred and used for development, the transfer property thereof is inferior so that the coarse particles form images at a position different from the target position (transfer dust).
In addition, mother particles obtained by the pulverization method or the polymerization method described above are typically subject to mixing treatment in which the mother particles are mixed with external additives such as inorganic particulates or organic particulates to improve fluidity, charging stability, lubricant property, and cleaning property.
However, subsequent to the mixing process, when the toner having a high chargeability passes through a screen, the toner is charged by the contact with the screen, which may lead to re-agglomeration of the toner. This is significant especially when the screen is made of metal with a small opening size. When toner particles that have just passed through a screen having an opening size of 65 μm or less is screened by another screen having the same opening size, the toner may not pass through but remain on the second screen. This results in reduction of productivity and agglomerated toner is used for development, resulting in occurrence of white spots (hollow defects) in a solid image portion.